Fly ash recovery



C. W. J. HEDBERG ET AL FLY ASH RECOVERY Filed Dec. 27, 1950 Us: umJwmote :Omd &

May 4, 1954 INVENTORS CARLMJ. HEDBERG HARRY A. WINTERMUTE A'ITUKNEYSPatented May 4, 1954 FLY ASH RECOVERY Carl W. J. Hedberg, Bound Brook,and Harry A.

Wintermute, Plainfield, N. J assignors to Research Corporation, NewYork, N. Y., a. corporation of New York Application December 27, 1950,Serial No. 202,974

2 Claims. 1

This invention relates to a method and apparatus for the recovery of flyash produced in the burning of pulverized coal.

Although electrical precipitators provide the most satisfactory meansfor the recovery of fly ash from combustion gases from boiler plantsburning powdered coal, the efficiency of the recovery frequently is lessthan desirable, particularly when anthracite coals are being burned orwhen a high efficiency of combustion is attained in the boiler.

It has now been found that the efficiency of collection of fly ash canbe substantially increased and the electrical operation of theprecipitators improved by adding to the fly ash containing combustiongases before they enter the electrical precipitators very finely dividedcoal particles. Preferably the major portion of the coal particles areless than 10 microns in size. The finely divided coal is added to thecombustion gases in an amount of preferably from about 10 to about 20%based on the fly ash content of the gases. This amount may be reduced inproportion to the amount of residual unburned coal present in the flyash. It is desirable to add the coal to the gases at such a point thatthe coal will be uniformly distributed in the gases at the time theyenter the precipitators and preferably at a point where the temperatureis below the temperature of substantial thermal decomposition of thecoal. A convenient source of the finely divided coal particles is thepowdered coal stream coming from the coal pulverizer of a powdered coalboiler, although it is desirable to separate out the more finely dividedportion of the powdered coal for the purpose of the invention.

The method and apparatus of the invention will be more particularlydescribed with reference to the accompanyin drawing which is adiagrammatic representation or" a powdered coal boiler system embodyingthe principles of the invention.

In the drawing, E is a powdered coal boiler supplied with powdered coalin suspension in air from pulverizer it through blower l2. Combustiongases from the boiler pass through an air preheater i3 and thence to afly ash collecting system shown by way of illustration as comprising amechanical collector [4 followed by electrical precipitators i5.Incoming air for combustion passes from exchanger l3 to the boiler. Inadapting the principles of the invention to this typical pulverized coalboiler system, there is provided a flue H3 leading a portion of thepowdered coal suspension from the blower to a centrifugal separator llwherein most of the coal particles over microns in size are separatedand returned to the blower through star valve l8 and conduit H9, or tothe pulverizer H by a suitable conduit to the pulverizer feed.

The finely divided coal particles are carried from the separator by ailue 20 which is shown opening into the combustion gas fiue between themechanical collector and the electrical precipitators. The suction onthe outlet of the separator ll due to the induced draft of the boilersystem and the pressure on the inlet of the separator from blower I2provides a pressure head adequate to carry the finely divided coalparticles to any desired point in the system. The finely divided coalmay be supplied to the combustion gases at any other convenient placeahead of the precipitators as between the heat exchanger and themechanical collector or ahead of the heat exchanger. It is, of course,necessary, in the latter case, that the major portion of the coalparticles be small enough to pass through the mechanical collector.

It is advantageous to provide a fiue 2| leading from the combustion airconduit to the pulverized coal take-off system to provide additional airfor carrying the finely divided coal to the combustion gas fiue.Suitable dampers for regulation of the fiow of air through flue 2! arealso advantageously provided.

The arrangement of apparatus and lines shown in the drawing is purelyillustrative and other arrangements may be used without departing fromthe principles of the invention.

\Ve claim:

1. In a powdered coal boiler system including a coal pulverizersupplying powdered coal to the boiler and an electrical precipitatorthrough which the combustion gases are passed for collection of fiy ash,means for transferring a portion of the powdered coal from thepulverizer to the combustion gas flue ahead of the electricalprecipitator.

2. In a powdered coal boiler system including a coal pulverizersupplying powdered coal to the boiler and an electrical precipitatorthrough which the combustion gases are passed for collection of fly ash,a separating device for separating a portion of the powdered coal fromthe pulverizer into a fraction containing a major portion of coalparticles less than 10 microns in size and means for supplying saidfraction of finely divided coal to the combustion gas flue ahead of theelectrical precipitator.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,329,237 Frisbie Jan. 27, 1920 1,329,817 Wolcott Feb. 3, 19201,331,225 Wolcott Feb. 17, 1920 1,343,482 Schmidt et a1. June 15, 19201,446,778 Witte Feb. 27, 1923 1,774,604 Lawrence Sept. 2, 1930 1,883,372Hedberg Oct. 18, 1932

